docs: def extract(self, inner_idx):
          """
          Extracts a component of the underlying ProbabilityCurves,
          specified by the index `inner_idx`.
          """
          out = self.__class__(self.shape_y, 1)
          for sid in self:
              curve = self[sid]
              array = curve.array[:, inner_idx].reshape(-1, 1)
              out[sid] = ProbabilityCurve(array)
          return out